Channel equalizer and digital television receiver using the same

ABSTRACT

Disclosed are a channel equalizer and digital television receiver using the same. The equalizer comprises a channel estimator, a channel distortion compensator and noise canceller. The channel estimator estimates an impulse response of a transmission channel from a received signal. The channel distortion compensator transforms and processes the received signal and the estimated impulse response. The noise canceller predicts and eliminates amplified noise generated during equalization.

[0001] This application claims the benefit of the Korean Application No.P2002-79963 filed on Dec. 14, 2002, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a channel equalizer used in afrequency domain and a digital television receiver using the same.

[0004] 2. Discussion of the Related Art

[0005] In a general digital communication system, a transmitter mapsdigital information such as voice, data and image onto symbols, convertseach symbol into an analog signal proportional to an amplitude or aphase corresponding to the symbol, and transmits the analog signal to areceiver through a transmission channel. The signal transmitted to thereceiver is interfered with adjacent signal during its passing throughthe transmission channel of multiple paths, so that the signal isdistorted very seriously. In order to restore the original signal from adistorted signal, an equalizer is essentially employed to compensate achannel. In general, the most widely used channel equalizer is adecision feedback equalizer (DFE) that uses LMS algorithm. When signalsare received through a multiple path channel, the DFE regards the paththrough which the signal having the largest energy is received as a mainpath, and regards the remaining paths as reflection paths through whichan inter-symbol interference (ISI) or ghost signals are received. Then,the DFE corrects and extracts the phase and the amplitude of only thesignal received through the main path and eliminates the signalsreceived through the remaining paths.

[0006]FIG. 1 illustrates a configuration of a general decision feedbackequalizer operating in a time domain, that is, a time domain equalizer.Referring to FIG. 1, a feed forward filter 101 removes the affection ofthe signals (pre-ghost signals) of paths, which are received before thesignal of the main path, and a feedback filter 102 removes the affectionof the signals (pre-ghost signals) of paths, which are received afterthe signal of the main path. An adder 105 adds the output of the feedforward filter 101 and the output of the feedback filter 102, andoutputs the sum of the outputs to a decision unit 103. The decision unit103 compares the output signal of the adder 105 with a predeterminedreference value to determine the output signal of the adder 105 to be atthe nearest signal level. Here, the output of the decision unit 103 isfed back to the feedback filter 102 and the control unit 104.

[0007] Accordingly, when the decision unit 103 made a exact decision,noises are eliminated from the output of the decision unit 103 and theoutput of the decision unit 103 is inputted to the feedback filter 102again. So, the noise is not amplified and the time domain equalizershown in FIG. 1 is usually better than the linear equalizers inperformance.

[0008] Also, if a decision error is negligible, the time domainequalizer can have similar performance to that of the maximum likelihoodsequence estimator (MLSE).

[0009] However, if channel distortion is too serious, the decision erroroccurs frequently on the decision value inputted to the feedback filter102 and the wrong decision value is infinitively looped in the feedbackfilter 102. So, the time domain equalizer can deteriorate in itsperformance. This situation is called error propagation situation. Ifthe main path is cut and only the signals passing through the reflectionpaths exist, or if the same signal is transmitted though different pathsfrom multiple antenna (this network is, so called, a single frequencynetwork (SFN), the energies of the signals received through the pathsare similar to one another so that it is unclear which signal of them isthe main signal. In other words, in case the locations of a main pathand a reflection path in the time domain equalizer are occasionallychanged, the time domain equalizer deteriorates in performance and framesynchronization changes frequently so that channel decoding performed ina rear stage of the equalizer is impossible.

[0010] In this situation, it is meaningless to distinguish a main signalfrom reflection signals and the DFE cannot equalize the signalscorrectly so that the DFE is not proper to multiple path and SFN channelcompensation.

SUMMARY OF THE INVENTION

[0011] Accordingly, the present invention is directed to a channelequalizer and a digital television receiver using the same, whichsubstantially obviate one or more problems due to limitations anddisadvantages of the related art.

[0012] An object of the present invention is to provide a channelequalizer and a digital television receiver using the same, in which theaffection of channel estimation value is minimized when a channel isequalized in frequency domain so that the channel equalizer and thedigital television receiver has constant equalization performance evenin case an estimation channel error exists or in the dynamic channelsituation that changes faster than appearance frequency of a trainingsignal.

[0013] Another object of the present invention is to provide a channelequalizer and a digital television receiver using the same, in whichfrequency domain equalization is performed using an initial coefficientof the frequency domain equalization for channel estimation valueobtained by means of a training sequence and using an LMS adaptivealgorithm for data duration so that the channel equalizer and thedigital television receiver has excellent performance in mobilereceiving environment.

[0014] Additional advantages, objects, and features of the inventionwill be set forth in part in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

[0015] To achieve these objects and other advantages and in accordancewith the purpose of the invention, as embodied and broadly describedherein, the present invention provides a channel equalizer. The channelequalizer estimates a channel by using a training signal, obtains afrequency response of an inverse channel from the estimated channel,uses the obtained frequency response as an initial coefficient of thefrequency domain equalizer, and performs frequency domain equalizationwith respect to a data period by using an LMS adaptive algorithm so thata constant equalization performance can be obtained even in case anestimation channel error exists or in the dynamic channel situation thatthe frequency is changed faster than appearance frequency of a trainingsignal.

[0016] Also, the inventive channel equalizer allows a robust receptionperformance to be exhibited even in a moving reception circumstance aswell as a fixed reception circumstance by overcoming equalizationfailure and synchronization failure of data frame resulted from theimpossible discrimination between a main path and a reflection pathunder an environment of multiple paths or a single frequency network(SFN), which are problems of the conventional equalizer.

[0017] In another aspect of the invention, a channel equalizer forrestoring an original signal from a digital television received signalthat past through a channel, comprises: a channel estimator forestimating an impulse response of a transmission channel from a receivedsignal having past through the transmission channel, thereby estimatingthe transmission channel; a channel distortion compensator forconverting the received signal and the impulse response of the estimatedtransmission channel into frequency domain signals, setting a reciprocalof the impulse response of the estimated transmission channel in afrequency domain as an initial coefficient, updating the coefficientscontinuously in data duration, compensating distortion of the receivedsignal converted into the frequency domain, and converting thecompensated received signal back into a time domain; and a noisecanceller for predicting a noise from an output of the channeldistortion compensator, the noise being amplified when equalized, andeliminating the amplified noise contained in the time domain signaloutputted from the channel distortion compensator.

[0018] The channel estimator detects a training period, calculating across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,and outputting the calculated cross-correlation as the impulse responseof the estimated transmission channel.

[0019] The channel estimator detects a training period, calculating across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,multiplying the cross-correlation and an inverse matrix of anauto-correlation matrix of the training signal, and outputting themultiplication result as the impulse response of the estimatedtransmission channel.

[0020] The channel estimator detects a training period, calculating across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,multiplying the cross-correlation and an inverse matrix of anauto-correlation matrix of the training signal, and outputting anaverage of the multiplication result and the impulse response of theestimated transmission channel of a previous frame as the impulseresponse of the estimated transmission channel.

[0021] The channel distortion compensator comprises: a first fastFourier transform (FFT) unit for converting the received signal from thetime domain into the frequency domain; a second FFT unit for convertingthe impulse response of the transmission channel estimated by thechannel estimator from the time domain into the frequency domain; a ROMfor in advance tabling and storing frequency responses corresponding toan inverse channel of the transmission channel of the frequency domain,and selectively outputting a frequency response corresponding to aninverse channel of the estimated transmission channel, which wasoutputted from the second FFT unit; and a frequency domain equalizer forreceiving the frequency response of the inverse channel outputted fromthe ROM, setting the frequency response as an initial coefficient,receiving a channel equalized signal fed back in data duration, updatingthe coefficient continuously, compensating the distortion of thereceived signal converted into the frequency domain, and converting thecompensated received signal back into the time domain.

[0022] The first FFT unit overlaps a received data block whose length isM and a previous data block, and converts the received data block intothe frequency domain.

[0023] The second FFT unit augments zeros to the impulse response of theestimated channel, whose length is M, so that a size of the augmentedimpulse response matches a size N of an FFT block, and converts theaugmented impulse response into the frequency domain.

[0024] The frequency domain equalizer comprises: a coefficient bank forreceiving the frequency response of the inverse channel outputted fromthe ROM, setting the frequency response as an initial coefficient forfrequency domain equalization, and storing and outputting coefficientsthat are updated continuously in the data duration; a first complexmultiplier for multiplying the received frequency domain signaloutputted from the first FFT unit and a coefficient outputted from thecoefficient bank and compensating the channel distortion contained inthe received frequency domain signal; an IFFT unit for converting thereceived frequency domain signal, which is outputted from the firstcomplex multiplier and from which distortion was compensated, back intothe time domain; a third FFT unit for receiving an error signal, adifference between an output of the IFFT unit and the signal from whicha noise was eliminated by the noise canceller, and converting the errorsignal into the frequency domain; a complex conjugate generator foroutputting complex conjugate values of the received frequency domainsignal outputted from the first FFT unit; a second complex multiplierfor multiplying an output of the third FFT unit and an output of thecomplex conjugate generator; a multiplier for multiplying an output ofthe second complex multiplier and a step size (α); and an adder foradding an output of the multiplier and a previous coefficient fed backfrom the coefficient bank thereby updating a coefficient, and outputtingthe updated coefficient to the coefficient bank.

[0025] The IFFT unit extracts only rear M samples from N signalsconverted into the time domain and outputs the extracted rear M samplesto the noise canceller.

[0026] The third FFT unit augments zeros to a front of the error signalwhose length is M so that a size of the augmented error signal matches asize N of an FFT block, and converts the augmented error signal into thefrequency domain.

[0027] The noise canceller comprises: a noise predictor for extractingonly colored noises from an output of the channel distortion compensatorby using the output of the channel distortion compensator and a decisionvalue of a signal from which an amplified noise is eliminated and whichis fed back, and predicting the noise amplified when equalized; and afirst subtracter for subtracting the noise predicted by the noisepredictor from the output of the channel distortion compensator, therebywhitening the noise.

[0028] The noise canceller further comprises: a decision unit connectedto an output terminal of the noise canceller, for outputting a decisionvalue nearest to a signal which is outputted from the noise cancellerand from which the amplified noise is eliminated; a multiplexer forfeeding the training signal back to the noise predictor in a trainingperiod and feeding back the decision value nearest to the signal fromwhich the noise was eliminated to the noise predictor in a dataduration; and a second subtracter for outputting as an error signal adifference between a signal outputted through the multiplexer and anoutput signal of the channel distortion compensator to the third FFTunit of the channel distortion compensator.

[0029] In another aspect of the present invention, a channel equalizerfor restoring an original signal from a digital television receivedsignal that past through a channel, comprises: a channel distortioncompensator for converting the received signal into a frequency domain,receiving a channel equalized signal fed back, updating coefficientscontinuously, compensating distortion of the received signal convertedinto the frequency domain, and converting the compensated receivedsignal back into a time domain; and a noise canceller for predicting anoise from an output of the channel distortion compensator, the noisebeing amplified when channel equalization is performed, eliminating theamplified noise contained in the time domain signal outputted from thechannel distortion compensator, and feeding the time domain signal backto the channel distortion compensator so as to update a coefficient.

[0030] The channel equalizer further comprises a channel estimatorpositioned at a front of the channel distortion compensator, forestimating an impulse response of a transmission channel from a receivedsignal having past through the transmission channel, converting theestimated impulse response into the frequency domain, and downloading areciprocal of the impulse response of the estimated transmission channelin a frequency domain as an initial coefficient of the channeldistortion compensator for the equalization in the frequency domain.

[0031] In another aspect of the present invention, a channel equalizerfor restoring an original signal from a digital television receivedsignal that past through a channel, the channel equalizer comprises: achannel estimator for estimating an impulse response of a transmissionchannel from a received signal having past through the transmissionchannel, thereby estimating the transmission channel; and a channeldistortion compensator for converting the received signal and theimpulse response of the estimated transmission channel into frequencydomain signals, setting a reciprocal of the impulse response of theestimated transmission channel in a frequency domain as an initialcoefficient, receiving a channel equalized signal fed back in dataduration, updating the coefficients continuously, compensatingdistortion of the received signal converted into the frequency domain,and converting the compensated received signal back into a time domain.

[0032] The channel equalizer further comprises a noise canceller forpredicting a noise from an output of the channel distortion compensator,the noise being amplified when equalized, and eliminating the amplifiednoise contained in the time domain signal outputted from the channeldistortion compensator.

[0033] In another aspect of the present invention, a digital televisionreceiver comprises: a demodulator for digitalizing a received signaldemodulating the digitalized signal into a base band signal; a channelestimator for estimating an impulse response of a transmission channelfrom an output signal of the demodulator; a channel distortioncompensator for converting the received base band signal and the impulseresponse of the estimated transmission channel into frequency domainsignals, setting a reciprocal of the impulse response of the estimatedtransmission channel in a frequency domain as an initial coefficient,receiving channel equalized data fed back in data duration, updating thecoefficients continuously, compensating distortion of the receivedsignal converted into the frequency domain, and converting thecompensated received signal back into a time domain; a noise cancellerfor predicting a noise from an output of the channel distortioncompensator, the noise being amplified when equalized, and eliminatingthe amplified noise contained in the time domain signal outputted fromthe channel distortion compensator; and an error compensator forcompensating a phase and an error of data outputted from the noisecanceller and outputting the compensated data for channel decoding.

[0034] Other objects, characteristics and advantages of the presentinvention will be clear through the description of the embodimentsreferring to the accompanied drawings.

[0035] It is to be understood that both the foregoing generaldescription and the following detailed description of the presentinvention are exemplary and explanatory and are intended to providefurther explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this application, illustrate embodiment(s) of theinvention and together with the description serve to explain theprinciple of the invention. In the drawings:

[0037]FIG. 1 illustrates a configuration of a conventional decisionfeedback equalizer operating in a time domain;

[0038]FIG. 2 illustrates a configuration of a channel equalizeroperating in a frequency domain according to an embodiment of thepresent invention;

[0039]FIGS. 3A to 3H illustrate frequency spectra to teach the operationof a amplified noise canceller shown in FIG. 2;

[0040]FIG. 4 is a detailed block diagram illustrating the noisecanceller shown in FIG. 2; and

[0041]FIG. 5 illustrates a configuration of a channel equalizeroperating in a frequency domain according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0042] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0043]FIG. 2 is a block diagram illustrating the entire configuration ofa channel equalizer of a digital television receiver according to thepresent invention. A channel equalizer comprises a channel estimator210, a channel distortion compensator 300 and a noise canceller 400. Thechannel estimator 210 estimates a transmission channel in a time domainby using a training signal. A channel distortion compensator 300converts the estimated transmission channel into a frequency domain,uses the frequency response of the inverse channel of the estimatedtransmission channel (converted) into the frequency domain as an initialcoefficient for frequency domain equalization, adaptively equalize thechannel-distorted data. The noise canceller 400 eliminates the noisecontained in the time domain output data of the channel distortioncompensator.

[0044] The channel distortion compensator 300 comprises a first fastFourier transform (FFT) unit 321, a second FFT unit 322, a ROM 323, anda frequency domain equalizer 330. The first FFT unit 321 converts thereceived signal from the time domain into the frequency domain. Thesecond FFT unit 322 converts the transmission channel estimated by thechannel estimator 210 from the time domain into the frequency domain.The ROM 323 in advance tables and stores frequency responsescorresponding to an inverse channel of the transmission channel of thefrequency domain, and outputs a frequency response corresponding to aninverse channel of the estimated transmission channel converted into thefrequency domain. The frequency domain equalizer 330 uses, as an initialcoefficient, the frequency response of the inverse channel outputtedfrom the ROM 323 when performing frequency domain equalization on theinput signal outputted from the first FFT 321, and adaptively equalizesthe distorted transmission channel signal for the data duration with theinitial coefficient by using the LMS algorithm and compensates thechannel distortion.

[0045] The frequency domain equalizer comprises a coefficient bank 338,a first complex multiplier 331, an IFFT unit 332, a third FFT unit 333,a complex conjugate generator 335, a second complex multiplier 334, amultiplier 336 and an adder 337. The coefficient bank 338 uses thefrequency response outputted from the ROM 323 as an initial coefficientand stores coefficients necessary to channel equalization. The firstcomplex multiplier 331 multiplies the frequency domain signal outputtedfrom the first FFT unit 321 and a coefficient outputted from thecoefficient bank 338 and compensates the channel distortion contained inthe frequency domain signal outputted from the first FFT unit 321. TheIFFT unit 332 converts the frequency domain signal, which is outputtedfrom the first complex multiplier and from which distortion wascompensated, back into the time domain. The third FFT unit 333 receivesan error signal, a difference between an output of the IFFT unit and thesignal from which a noise was eliminated by the noise canceller, andconverting the error signal into the frequency domain. The complexconjugate generator 335 outputs complex conjugate values of thefrequency domain signal outputted from the first FFT unit 321. Thesecond complex multiplier 334 multiplies an output of the third FFT unit333 and an output of the complex conjugate generator 335. The multiplier336 multiplies an output of the second complex multiplier 334 and a stepsize (α). The adder 337 adds an output of the multiplier 336 and anoutput of the coefficient bank 338 to update a coefficient, and storesthe updated coefficient in the coefficient bank 338.

[0046] In the present invention configured as described above, the inputsignal inputted to the receiver can be presented as the followingequation. $\begin{matrix}{{y(n)} = {{\sum\limits_{k}^{\quad}\quad {{h(k)} \cdot {x\left( {n - k} \right)}}} + {w(n)}}} & {{Equation}\quad 1}\end{matrix}$

[0047] where x(n) is a transmitted signal, h(n) is an impulse responseof discrete equivalent channel, w(n) is a white noise, and y(n) is theinput signal inputted to the receiver.

[0048] As shown in FIG. 2, the channel estimator 210 estimates animpulse response h(n) of the discrete equivalent channel through whichan original signal x(n) is considered to have past, and outputs anestimated value ĥ(n) of finite impulse response of the channel to thesecond FFT unit 322 of the channel distortion compensator 300.

[0049] Here, the simplest channel estimation method of the channelestimator 210 is called a simple correlation method (SCM). In SCM, thetraining signal added periodically to the transmitted signal is supposedto be a white signal and the training period is detected. Then,cross-correlation of the training signal that past through the channelfor the training period and the training signal that the receiver knowsis obtained and the cross-correlation is outputted as the estimatedchannel impulse response.

[0050] The above-mentioned SCM is so simple to be implemented by simplehardware. However, when the training signal does not have a white signalproperty, the estimation error is large. Furthermore, the larger thechannel estimated area is, the more the estimation is influenced by thedata on both sides of the training signal. So, it is impossible toexactly estimate a channel.

[0051] Meanwhile, in the least square method (LSM) known to be moreexact method, compared with SCM, it is possible to exactly estimate achannel even if the train signal does not have white signal property. Inother word, the training period is detected and the cross-correlationvalue p of the training signal that past through the channel for thetraining period and the training signal that the receiver knows isobtained. The auto-correlation matrix R of the training signal isobtained. The matrix calculation of R⁻¹·P is performed to remove theauto-correlation existing in the cross-correlation value p of thereceived signal and the original training signal. So, it is possible tomore exactly estimate a channel. In the LSM, the auto-correlation matrixR is an identity matrix when the training signal is a white signal. So,in that case, the SCM and the LSM have the same result. In the LSM,compared with the SCM, it is possible to more exactly estimate a channelowing to complex implementation. When the channel estimation areabecomes larger, LSM is influenced by data as much as the SCM.

[0052] To solve this problem, average LS estimation method is suggested.In other words, in the average LS estimation method, the channelestimation value of the LSM is properly filtered and averaged so thatthe influence from the data is minimized. For example, the trainingperiod is detected and the cross-correlation value of the trainingsignal that past through the channel for the training period and thetraining signal preset by the receiver is calculated. Thecross-correlation value p and an inverse matrix R⁻¹ of theauto-correlation matrix are multiplied and the average of themultiplication result R⁻¹·R and the stored impulse response of theestimation channel of the previous frame is outputted as the impulseresponse of the estimated channel.

[0053] The channel estimator 210 may include any one of the inexactchannel estimators of the SCM or the LSM as well as average LSestimation method.

[0054] As described above, a finite impulse response estimated valueĥ(n) of the channel estimated by the channel estimator 210 by using anyone of the channel estimation methods is outputted to the second FFTunit 322 of the channel distortion compensator 300.

[0055] In other words, since the channel equalization is performed inthe frequency domain, the second FFT unit 322 converts a time domainimpulse response ĥ(n) of an estimated channel into the frequency domain.The impulse response Ĥ(w) of the channel, which was converted into thefrequency domain, is outputted to the ROM 323.

[0056] Here, since the block size of the second FFT unit 322 is N andthe number of the actually estimated frequency responses is M, thesecond FFT unit 322 augments M zeros to M frequency responses to expandthe responses to the size of 2M=N and converts the expanded responsesinto a frequency domain.

[0057] The ROM 323 stores the table containing the reciprocal values ofthe input values. The reciprocal value Ĥ(w)⁻¹ of the impulse responseĤ(w) of the channel, which was converted into the frequency domain, isselected by the ROM 323 and then outputted to the coefficient bank 338of the frequency domain equalizer 330.

[0058] Here, though the frequency response of the estimated channel hasfrequency bin that has zero, since the reciprocal is obtained throughthe ROM 323, the frequency response can be limited to a finite value, sothat divergence can be prevented.

[0059] The frequency response of the inverse channel obtained by the ROM323 is downloaded to the coefficient bank 338 that has a storage of FFTblock size, and is used as initial coefficient for frequency domainequalization on a new coming data block.

[0060] Here, the coefficients may be downloaded whenever the trainingsignal appears, or may be early downloaded only once and then adaptiveequalization is performed continuously. One of the above-mentioned twomethods may be selected or both of them may be used. It depends on adesigner who implements the method.

[0061] The process in which the frequency domain equalizer 330 performsadaptive channel equalization in the frequency domain by using an LSMadaptive algorithm will be described.

[0062] In the channel equalization process using the LMS adaptivealgorithm in the frequency domain, there are a process in which timedomain linear convolution is replaced with a circular convolution byusing FFT and duplicate storing method is used so that the result of thesubstituted circular convolution can be the same as that of the linearconvolution, and a process in which filter coefficients are updatedblock by block in the frequency domain by using an LMS algorithm.

[0063] In other words, the conventional time domain equalizer outputs anoutput signal whenever an input signal (or a symbol) appears. The errorsignal is obtained from the output to update the coefficient used toequalize a next symbol. In the method of updating the coefficient blockby block, all the input data of a block are equalized using the samecoefficients. The coefficient to be used for the next input signal blockis updated from an error signal block of the same size.

[0064] First, when a digital broadcast signal is received, the digitalbroadcast signal is demodulated, digitalized and shifted to the baseband. The first FFT unit 321 converts the received signal y(n) that wasshifted to the base band to the frequency domain, and the signal Y(w) isoutputted to the complex multiplier 331 of the frequency domainequalizer 330.

[0065] Here, when first FFT unit 321 converts the time domain inputsignal into the frequency domain, it should noticed that the linearconvolution in the time domain is replaced by the circular convolutionand so the duplicate storing method should be used so as to match theresults of the linear convolution and the circular convolution.Accordingly, note that a block for FFT conversion should be configuredso that the current data can overlap the previous data.

[0066] For example, supposing that the size of the FFT conversion blockis N and N=2M (overlap ratio=50%), the k-th block is configured asequation 2.

[0067] Equation 2

y _(k) =[y(k*M−M) . . . y(k*M−1)y(k*M) . . . y(k*M+M−1)]^(T)

[0068] Accordingly, the time domain data which has a size N and is anoverlapped input signal block is converted into N frequency bins by theFFT unit 321. The first complex multiplier 331 multiplies N frequencybins and the frequency bins of the same size N stored in the coefficientbank 338 by bin-by-bin multiplication.

[0069] Here, the bin-by-bin multiplication means multiplication of thesame frequency bins. The bin-by-bin multiplication of two signals in thefrequency domain corresponds to the circular convolution. Themultiplication result of the first complex multiplier 331 corresponds tothe equalized frequency bins. The equalized frequency bins are outputtedto the IFFT unit 332 to be converted back into the time domain.

[0070] Supposing that the signal converted back into the time domain isz(n), z(n) is N time domain samples. If the overlap ratio is 50% forexample, the M front samples contain aliasing components resulted fromcircular convolution, and the M rear samples include non-aliasingcomponents as the result of the linear convolution.

[0071] Accordingly, though not shown in FIG. 2, an extractor forextracting M samples as the result of the linear convolution from the Nsamples is implicitly added to the rear of the IFFT unit 332.

[0072] The M time domain signal z(n) obtained as described above can bethe sum of the value {circumflex over (x)}(n) which was equalized withequalization coefficients obtained from the previous block and theamplified colored noise v(n). The amplified colored noise of the signalis whitened by the amplified noise canceling filter of the noisecanceller 400. The whitened value is the final output of the equalizerand inputted to an phase tracker block. The phase tracker compensatesthe phase and the error of the data and outputs the compensated data tothe FEC decoder for an error correction.

[0073] The final output from which an amplified noise is eliminated isinputted to the decision unit 341 so as to generate an error forupdating new equalization coefficient.

[0074] The decision unit 341 outputs the decision values nearest to theoutput of the equalizer to the multiplexer 343. The multiplexer 343 is akind of selector. The multiplexer 343 selectively outputs a trainingsequence of a training signal generator 342 in a training signalduration and the output value of the decision unit 341 in a dataduration, as an ideal value or a reference value. In other words, theoutput of the multiplexer 343 is outputted to the noise canceller 400and the subtracter 344.

[0075] The subtracter 344 obtains the difference between the referencevalue outputted through the multiplexer 343 and the output value of thefrequency domain equalizer 330. The difference is the error signal. Thiserror signal is inputted to the third FFT unit 333 and converted intothe frequency domain.

[0076] Here, since the third FFT unit 333 has the block size of N and Merror signals are actually generated, M zeros are augmented to a frontof the M error signals to expand the augmented error signal to have thesize of 2M=N and the expanded error signal to the third FFT unit 333.

[0077] The frequency domain error signal E(w) outputted from the thirdFFT unit 333 is outputted to the second complex multiplier 334. Thecomplex conjugate of the received frequency domain signal Y(w) outputtedfrom the first FFT unit 321 is outputted to the second complexmultiplier 334 though a complex conjugate generator 335.

[0078] For example, if the received frequency domain signal outputtedfrom the first FFT unit 321 is a+jb, the complex conjugate of thereceived frequency domain signal outputted from the complex conjugategenerator 335 is a−jb. In other words, the complex number whose realpart ‘a’ has a same sign and whose imaginary part ‘jb’ has an oppositesign is a complex conjugate.

[0079] The second complex multiplier 334 multiplies the output of thethird FFT unit 333 and the output of the complex conjugate generator 335and outputs the multiplication result to the multiplier 336. Here, thecalculation of the second complex multiplier 334 corresponds to the timedomain circular cross-correlation value.

[0080] The frequency domain signal of the circular cross-correlationvalue obtained above is inputted to the multiplier 336 and multipliedwith step size (α) to be outputted to the adder 337.

[0081] The adder 337 adds the output of the multiplier 336 and theexisting coefficient outputted from the coefficient bank 338 to updatethe coefficient. In other words, the output of the multiplier 336 isadded to the existing coefficient stored in the coefficient bank 338 sothat the coefficient for frequency domain equalization of the next blockis regenerated. Similarly, in the coefficient updating calculation ofthe adder 337, the same frequency bins should be added to update thecoefficient.

[0082] The coefficient updating method described until now is alsocalled ‘unconstrained coefficient updating method’. Because of thesimplicity of the structure, the unconstrained coefficient updatingmethod is widely used in spite of its slow convergence speed. Here, theterm “unconstrained” is originated from the ignorance of a restrictioncondition in that the length of the coefficient should exist by thenumber of M.

[0083] The unconstrained coefficient updating method is an embodiment ofthe present invention. In the present invention, the coefficient can beupdated in the constrained coefficient updating method. In other words,the output of the second complex multiplier 334 is converted into thetime domain through the IFFT unit. The M front coefficient updatingcomponents is obtained and the M rear components are replaced withzeros. The changed output of the IFFT unit is converted in FFTconversion and the converted result is used as frequency domaincoefficient updating component. This method is called constrainedcoefficient updating method. However, if a large amount of latency iscaused in implementing FFT or IFFT as a hardware, the delay value in afeedback loop becomes larger, so that the performance may be loweredcompared with the unconstrained coefficient updating method. Therefore,this method needs an attention during its use.

[0084] Hereinafter, a process of eliminating an amplified noise by usingthe noise predictor of the noise canceller 400 will be described indetail.

[0085] First, the transmission signal x(n) is converted into an analogsignal, and the analog signal is modulated and then transmitted to thereceiver through the channel. An impulse response of a discreteequivalent channel representing the whole procedures of re-sampling theresulting signal at a symbol rate after a carrier and symbol recovery isprovided as “h(n)”. At this point, as described above, a received signaly(n) can be expressed by a convolution of the transmission signal x(n)and the impulse response h(n). Herein, the process of eliminating theamplified noise from the output of the equalizer will be describedconceptually and then in detail. For the sake of convenience, it isassumed that the impulse response h(n) of the discrete equivalentchannel is given by the following Equation 3.

[0086] [Equation 3]

h(n)=δ(n)+δ(n−1)

[0087] Since the frequency spectrum X(w) of the transmission signal hasno distortion, the frequency spectrum X(w) is represented like FIG. 3Aand the white noise added to the reception signal is represented likeFIG. 3B. At this point, a frequency response H(w) of the channel givenby the Equation 3 is represented like FIG. 3C and a frequency responseY(w) of the channel passed signal y(n) is represented like FIG. 3D. Ifit is assumed that the channel estimator 210 accurately estimates thechannel, a frequency response Ĥ(w) of the estimated channel isrepresented like FIG. 3E and a spectrum of the converged frequencydomain equalizer coefficient for channel distortion compensation, iĤ(w)is represented like FIG. 3F. Accordingly, a multiplication of Ŷ(w) andiĤ(w) is represented like FIG. 3G. It can be seen from FIGS. 3A to 3Gthat the output of the equalizer consists of the recovered originalsignal and a colored noise signal that is amplified while a signalpasses through the equalizer. Accordingly, the noise canceller 400 ofthe present invention estimates the colored noise and subtracts theestimated colored noise from the actual colored noise. This is a kind ofa noise-whitening filter for whitening only noise, as shown in FIG. 3H.

[0088] Hereinafter, an operation of the noise canceller 400 will bedescribed in detail.

[0089] As described above, assuming that the frequency domainequalization is completely achieved in the channel distortioncompensator 300, the input signal of the noise canceller 400 consists ofa sum of the original signal and the colored noise. In other words, ifthe input signal of the noise canceller 400 is q(n), q(n) is given bythe following Equation 4.

[0090] [Equation 4]

q(n)=x(n)+v(n)=x(n)+Σh ⁻¹(k)w(n−k)

[0091] where, x(n) is an ideally equalized original signal and v(n) is acolored noise that is generated by a convolution of an impulse responseof an inverse channel and a white noise w(n) added in a reception.

[0092] Accordingly, using the fact that v(n) is correlative with aprevious value, the noise predictor 410 of the noise canceller 400obtains {circumflex over (v)}(n) that is forward-predicted by projectingthe colored noise v(n) on a plane spanned by the set of random vectors,{v(n−1), v(n−2), . . . , v(n−L)}. If the subtracter 420 subtracts thepredicted {circumflex over (v)}(n) from v(n), the elimination of theamplified noise is achieved. In other words, the noise amplified in theequalization can be eliminated by whitening v(n) through the subtractionof the predicted value {circumflex over (v)}(n) from v(n).

[0093]FIG. 4 is a detailed block diagram of the noise canceller 400. Thenoise canceller 400 includes: a first subtracter 401 for extracting onlythe colored noise v(n) by subtracting an output of a MUX 343 from anoutput of the channel distortion compensator 300, in which themultiplexer 343 selects a training sequence during a training signalperiod and a decision value of a noise-eliminated signal during a dataperiod, respectively; a noise predictor 410 for delaying an output ofthe first subtracter 401 in sequence and predicting v(n) by using thedelayed values v(n−1), . . . , v(n−L) to thereby generate {circumflexover (v)}(n); and a second subtracter 420 for whitening noise bysubtracting the predicted noise {circumflex over (v)}(n) from an outputq(n) of the channel distortion compensator 300, and outputting theequalized data with whitened noise to the FEC and the decision unit 341.Here, the decision unit 341 selects a decision value that is closest toa signal whose amplified noise is eliminated by the second subtracter420 of the noise canceller 400, that is, a signal whose noise iswhitened, and then outputs the selected decision value to themultiplexer 343. The decision unit 341, the training signal generator342 and the multiplexer 343 are a block shared by both the channeldistortion compensator 300 and the noise canceller 400.

[0094] A third subtracter 402 and a delay unit 403, which are notdescribed in FIG. 4, are provided for controlling a coefficient updatingof the noise predictor 410. The third subtracter 402 calculates adifference between the output of the first subtracter 401 and the outputof the noise predictor 410 and then outputs the calculated difference tothe delay unit 403. The delay unit 403 delays the inputted difference byunit time and then outputs the delayed signal to the respectivemultipliers of the noise predictor 410.

[0095] Referring to FIG. 4, the output q(n) of the channel distortioncompensator 300 includes the original signal x(n) and the colored noisev(n) together, as can be seen in Equation 4. The signal q(n) isoutputted to the first and second subtracters 401 and 420 of the noisecanceller 400.

[0096] The first subtracter 401 extracts only the colored noise signalv(n) by subtracting the original signal from the output q(n) of thechannel distortion compensator 300. Here, the original signal is asignal outputted through the multiplexer 343.

[0097] At this point, when the multiplexer 343 outputs the originalsignal to the first subtracter 401, it selectively outputs the trainingsequence during the training period and the decision value of thenoise-eliminated signal during the data period.

[0098] The colored noise signal v(n) extracted by the first subtracter401 is inputted to a first delay unit of a serial configuration of thenoise predictor 410. Each multiplier multiplies an output of each delayunit of the noise predictor 410 by the coefficient of predictor. Then,the results of the multipliers are added by the adder and outputted tothe second adder 420. At this point, if the coefficient of the noisepredictor 410 exists after the signal passes through the first delayunit, the output {circumflex over (v)}(n) of the noise predictor 410 isgiven by Equation 6, not Equation 5. $\begin{matrix}{{\hat{v}(n)} = {\sum\limits_{k = 0}^{L}\quad {p_{k}{v\left( {n - k} \right)}}}} & \left\lbrack {{Equation}\quad 5} \right\rbrack\end{matrix}$

$\begin{matrix}{{\hat{v}(n)} = {\sum\limits_{k = 1}^{L}\quad {p_{k}{v\left( {n - k} \right)}}}} & \left\lbrack {{Equation}\quad 6} \right\rbrack\end{matrix}$

[0099] where, p_(k) is a k-th coefficient of the noise predictor 410 andL is an order of the noise predictor 410.

[0100] A cost function J is defined as a mean square prediction error asfollows: $\begin{matrix}{{J \equiv {E\left\{ {e(n)}^{2} \right\}}} = {{E\left\{ \left( {{v(n)} - {\hat{v}(n)}} \right)^{2} \right\}} = {E\left\{ {{v(n)} - {\sum\limits_{k = 1}^{L}\quad {p_{k}{v\left( {n - k} \right)}^{2}}}} \right\}}}} & \left\lbrack {{Equation}\quad 7} \right\rbrack\end{matrix}$

[0101] where, E is an operation of calculating a probable expectationand e(n) is a prediction error.

[0102] If the cost function J is differentiated with respect to P_(k) inorder to calculate P_(k) (k=1, 2, . . . , L) that minimize the costfunction J, the result is given by Equation 8. $\begin{matrix}{\frac{\partial J}{\partial p_{k}} = {{{- 2} \cdot E}\left\{ {{e(n)} \cdot {v\left( {n - k} \right)}} \right\}}} & \left\lbrack {{Equation}\quad 8} \right\rbrack\end{matrix}$

[0103] If an instantaneous value is used instead of the probableexpectation in order to update coefficient using LMS algorithm, theresult is given by Equation 9.

[0104] [Equation 9]

E{e(n)·v(n−k)}≈e(n)·v(n−k)

[0105] Accordingly, if P_(k)(n) is a k-th prediction coefficient that isupdated at n-th time, the updating equation of coefficient with respectto time is given by Equation 10.

[0106] [Equation 10]

p _(k)(n+1)=p _(k)(n)+μ·e(n)·v(n−k), (k=1, 2, . . . , L)

[0107] The output {circumflex over (v)}(n) predicted using the updatedcoefficient is directly inputted to the second subtracter 420, thenwhitening the noise of the signal q(n). Here, the coefficient is updatedunder a control of the third subtracter 402 and the delay unit 403.

[0108] Accordingly, the output r(n) of the noise canceller 400 is givenby Equation 11.

[0109] [Equation 11]

r(n)=x(n)+{circumflex over (w)}(n)=x(n)+(v(n)−{circumflex over (v)}(n))

[0110] where, ŵ(n) represents a whitened noise. The output of the noisecanceller 400 is a signal whose channel is equalized and whose noise iseliminated, and a signal that is almost identical to the originalsignal.

[0111] Accordingly, since the value of Equation 11 can be consideredidentical to the decision value of the decision unit or the originalsignal, the performance of the noise predictor 410 is not degraded evenif the noise predictor 410 is operated using only the decided data valueat the input terminal without using the training signal.

[0112]FIG. 5 is a block diagram of a channel equalizer according toanother embodiment of the present invention, in which the channelestimator is not used.

[0113] Referring to FIG. 5, the channel equalizer in the frequencydomain includes a channel distortion compensator 600 and a noisecanceller 400. In FIG. 5, the channel estimator and the FFT forconverting the transmission channel estimated by the channel estimatorinto the frequency domain are not used and the operations of the otherblocks are equal to those described in FIG. 2.

[0114] In other words, the received signal y(n) is converted into thefrequency domain signal in a first FFT unit 610 and then outputted to acomplex multiplier 331 and a complex conjugate generator 335. Acoefficient bank 338 stores a coefficient outputted from an adder 337.The stored coefficient is outputted to the complex multiplier 331 andfed back to the adder 337 in order to update the coefficient at the sametime.

[0115] Meanwhile, according to another embodiment of the presentinvention, the channel equalizer can be also configured with only thechannel estimator and the channel distortion compensator in thefrequency domain. In other words, in this case, the noise canceller isnot used. At this point, the operations of the channel estimator and thechannel distort compensator are equal to those described in FIG. 2.

[0116] The channel estimator for use in the digital TV receiveraccording to the present invention has following effects.

[0117] First, since the channel estimator of the present invention has aserially-connected configuration of the linear equalizer and theamplified-noise canceller, it does not undergo an error propagationsituation occurring in the time-domain DFE. Therefore, a stableequalization can be achieved even at extreme multiple path channels orchannel situation such as SFN.

[0118] Second, the channel is estimated using the training signal andthe frequency response of the inverse channel is obtained using theinformation on the estimated channel. The corresponding value is used asan initial coefficient of the frequency-domain equalizer. Theequalization is performed using the LMS adaptive algorithm during thedata period. Therefore, a consistent performance can be obtainedregardless of the estimated channel error.

[0119] Third, since the accuracy of the channel estimator does notgreatly influence the performance of the equalizer, it is possible touse a simple channel estimator using a single correlation value. As aresult, hardware for the channel estimator can be reduced in size.

[0120] Fourth, since the inverse channel information of the estimatedchannel is used only as the frequency-domain equalization coefficientand the coefficient continues to be updated during the data period usingthe LMS adaptive algorithm, the channel distortion compensation can beachieved with respect to the time-varying channel, i.e., the channelthat is varied between the training signals, or a dynamic ghost channel.Therefore, a good performance can be obtained in a mobile receptionenvironment.

[0121] Fifth, in situation such that the positions of the main path andthe reflective path are changed occasionally, the time-domain equalizersuffers from the degradation in the equalization performance. However,the frequency-domain equalizer using the channel estimator of thepresent invention does not cause such degradation. Therefore, the framesynchronization of the output of the equalizer is always constant, thusmaking it possible to maintain the frame synchronization securely.

[0122] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A channel equalizer for restoring an originalsignal from a digital television received signal that past through achannel, the channel equalizer comprising: a channel estimator forestimating an impulse response of a transmission channel from a receivedsignal having past through the transmission channel; a channeldistortion compensator for converting the received signal and theestimated impulse response of the transmission channel into frequencydomain signals, setting a reciprocal of the estimated impulse responseof the transmission channel in a frequency domain as an initialcoefficient, receiving a channel equalized signal fed back in a dataduration, calculating the error between equalized signal and decisionvalue of the same equalized signal, updating the coefficients by usingthe error between equalized signal and decision value of the sameequalized signal continuously, compensating distortion of the receivedsignal converted into the frequency domain, and converting thecompensated received signal back into a time domain; and a noisecanceller for predicting a noise from an output of the channeldistortion compensator, the noise being amplified when equalized, andeliminating the amplified noise contained in the time domain signaloutputted from the channel distortion compensator.
 2. The channelequalizer of claim 1, wherein the channel estimator detects a trainingperiod, calculates a cross-correlation between a training signal pastthrough the channel during the training period and a training signalpreset by a receiver, and outputs the calculated cross-correlation asthe impulse response of the estimated transmission channel.
 3. Thechannel equalizer of claim 1, wherein the channel estimator detects atraining period, calculates a cross-correlation between a trainingsignal past through the channel during the training period and atraining signal preset by a receiver, multiplies the cross-correlationand an inverse matrix of an auto-correlation matrix of the trainingsignal, and outputs the multiplication result as the impulse response ofthe estimated transmission channel.
 4. The channel equalizer of claim 1,wherein the channel estimator detects a training period, calculates across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,multiplies the cross-correlation and an inverse matrix of anauto-correlation matrix of the training signal, and outputs an averageof the multiplication result and the estimated impulse response of thetransmission channel of a previous frame as the estimated impulseresponse of the transmission channel of the current frame.
 5. Thechannel equalizer of claim 1, wherein the channel distortion compensatorcomprises: a first fast Fourier transform (FFT) unit for converting thereceived signal from the time domain into the frequency domain; a secondFFT unit for converting the impulse response of the transmission channelestimated by the channel estimator from the time domain into thefrequency domain; a ROM for in advance tabling and storing frequencyresponses corresponding to an inverse channel of the transmissionchannel of the frequency domain, and selectively outputting a frequencyresponse corresponding to an inverse channel of the estimatedtransmission channel, which was outputted from the second FFT unit; anda frequency domain equalizer for receiving the frequency response of theinverse channel outputted from the ROM, setting the frequency responseof the inverse channel as an initial coefficient, receiving a channelequalized signal fed back in data duration, calculating the errorbetween equalized signal and decision value of the same equalizedsignal, updating the coefficients by using the error between equalizedsignal and decision value of the same equalized signal continuously,compensating the distortion of the received signal converted into thefrequency domain, and converting the compensated received signal backinto the time domain.
 6. The channel equalizer of claim 5, wherein thefirst FFT unit overlaps a received data block whose length is M and aprevious data block, and converts the received data block into thefrequency domain.
 7. The channel equalizer of claim 5, wherein thesecond FFT unit augments zeros to the impulse response of the estimatedchannel, whose length is M, so that a size of the augmented impulseresponse matches a size N of an FFT block, and converts the augmentedimpulse response into the frequency domain.
 8. The channel equalizer ofclaim 5, wherein the frequency domain equalizer comprises: a coefficientbank for receiving the frequency response of the inverse channeloutputted from the ROM, setting the frequency response of the inversechannel as an initial coefficient for frequency domain equalization, andstoring and outputting coefficients that are updated continuously in thedata duration; a first complex multiplier for multiplying the receivedfrequency domain signal outputted from the first FFT unit and acoefficient outputted from the coefficient bank and compensating thechannel distortion contained in the received frequency domain signal; anIFFT unit for converting the received frequency domain signal, which isoutputted from the first complex multiplier and from which distortionwas compensated, back into the time domain; a third FFT unit forreceiving an error signal, a difference between an output of the IFFTunit and the signal from which a noise was eliminated by the noisecanceller, and converting the error signal into the frequency domain; acomplex conjugate generator for outputting complex conjugate values ofthe received frequency domain signal outputted from the first FFT unit;a second complex multiplier for multiplying an output of the third FFTunit and an output of the complex conjugate generator; a multiplier formultiplying an output of the second complex multiplier and a step size(α); and an adder for adding an output of the multiplier and a previouscoefficient fed back from the coefficient bank thereby updating acoefficient, and outputting the updated coefficient to the coefficientbank.
 9. The channel equalizer of claim 8, wherein the frequencyresponse of the inverse channel is inputted to the coefficient bankwhenever the training signal is inputted.
 10. The channel equalizer ofclaim 8, wherein the frequency response of the inverse channel is earlyinputted only once to the coefficient bank, and then channel equalizeddata is fed back to update the coefficient.
 11. The channel equalizer ofclaim 8, wherein the IFFT unit extracts only rear M samples from Nsignals converted into the time domain and outputs the extracted rear Msamples to the noise canceller.
 12. The channel equalizer of claim 8,wherein the third FFT unit augments zeros to a front of the error signalwhose length is M so that a size of the augmented error signal matches asize N of an FFT block, and converts the augmented error signal into thefrequency domain.
 13. The channel equalizer of claim 8, wherein thenoise canceller comprises: a noise predictor for extracting only colorednoises from an output of the channel distortion compensator by using theoutput of the channel distortion compensator and a decision value of asignal from which an amplified noise is eliminated and which is fedback, and predicting the noise amplified during equalization; and afirst subtracter for subtracting the noise predicted by the noisepredictor from the output of the channel distortion compensator, therebywhitening the noise.
 14. The channel equalizer of claim 13, wherein thenoise canceller further comprises: a decision unit connected to anoutput terminal of the noise canceller, for outputting a decision valuenearest to a noise canceller output signal that the amplified noise iseliminated; a multiplexer for selectively feeding back the trainingsignal during a training period and a decision value of the signal thatthe noise is eliminated during a data period to the noise predictor; anda second subtracter for outputting as an error signal a differencebetween a signal outputted through the multiplexer and an output signalof the channel distortion compensator to the third FFT unit of thechannel distortion compensator.
 15. A channel equalizer for restoring anoriginal signal from a digital television received signal that pastthrough a channel, the channel equalizer comprising: a channeldistortion compensator for converting the received signal into afrequency domain, receiving a channel equalized signal fed back,calculating the error between equalized signal and decision value of thesame equalized signal, updating the coefficients by using the errorbetween equalized signal and decision value of the same equalized signalcontinuously, compensating distortion of the received signal convertedinto the frequency domain, and converting the compensated receivedsignal back into a time domain; and a noise canceller for predicting anoise from an output of the channel distortion compensator, the noisebeing amplified when channel equalization is performed, eliminating theamplified noise contained in the time domain signal outputted from thechannel distortion compensator, and feeding the time domain signal backto the channel distortion compensator so as to update a coefficient. 16.The channel equalizer of claim 15, wherein the channel distortioncompensator comprises: a first fast Fourier transform (FFT) unit forconverting the received signal from the time domain into the frequencydomain; a coefficient bank for storing and outputting coefficients thatare updated continuously in the data duration; a first complexmultiplier for multiplying the received frequency domain signaloutputted from the first FFT unit and a the coefficient outputted fromthe coefficient bank and compensating the channel distortion containedin the received frequency domain signal; an IFFT unit for converting thereceived frequency domain signal, which is outputted from the firstcomplex multiplier and from which distortion was compensated, back intothe time domain; a third FFT unit for receiving as an error signal adifference between an output of the IFFT unit and the signal from whicha noise was eliminated by the noise canceller, and converting the errorsignal into the frequency domain; a complex conjugate generator foroutputting complex conjugate values of the received frequency domainsignal outputted from the first FFT unit; a second complex multiplierfor multiplying an output of the third FFT unit and an output of thecomplex conjugate generator; a multiplier for multiplying an output ofthe second complex multiplier and a step size (α); and an adder foradding an output of the multiplier and a previous coefficient fed backfrom the coefficient bank thereby updating a coefficient, and outputtingthe updated coefficient to the coefficient bank.
 17. The channelequalizer of claim 16, wherein the first FFT unit overlaps a receiveddata block whose length is M and a previous data block, and converts thereceived data block into the frequency domain.
 18. The channel equalizerof claim 16, wherein the IFFT unit extracts only rear M samples fromsignals converted into the time domain and outputs the extracted rear Msamples to the noise canceller.
 19. The channel equalizer of claim 16,wherein the third FFT unit augments zeros to a front of the error signalwhose length is M so that the size of the augmented error signal matchesthe size N of an FFT block, and converts the augmented error signal intothe frequency domain.
 20. The channel equalizer of claim 15, wherein thenoise canceller comprises: a noise predictor for extracting only colorednoise from an output of the channel distortion compensator by using theoutput of the channel distortion compensator and a decision value of asignal from which an amplified noise is eliminated and which is fedback, and predicting the noise amplified during the equalization; and afirst subtracter for subtracting the noise predicted by the noisepredictor from the output of the output of the channel distortioncompensator, thereby whitening the noise.
 21. The channel equalizer ofclaim 20, wherein the noise canceller further comprises: a decision unitconnected to an output terminal of the noise canceller, for outputting adecision value nearest to a signal that the amplified noise iseliminated from the noise canceller; a multiplexer for selectivelyfeeding back the training signal during a training period and a decisionvalue of the signal that the noise is eliminated during a data period tothe noise predictor; and a second subtracter for outputting as an errorsignal a difference between a signal outputted through the multiplexerand an output signal of the channel distortion compensator to the thirdFFT unit of the channel distortion compensator.
 22. The channelequalizer of claim 15, further comprising: a channel estimatorpositioned at a front of the channel distortion compensator, forestimating an impulse response of a transmission channel from a receivedsignal having past through the transmission channel, converting theestimated impulse response into the frequency domain, and downloading areciprocal of the estimated impulse response of the transmission channelin a frequency domain as an initial coefficient of the channeldistortion compensator for the equalization in the frequency domain. 23.A channel equalizer for restoring an original signal from a digitaltelevision received signal that past through a channel, the channelequalizer comprising: a channel estimator for estimating an impulseresponse of a transmission channel from a received signal having pastthrough the transmission channel; and a channel distortion compensatorfor converting the received signal and the estimated impulse response ofthe transmission channel into frequency domain signals, setting areciprocal of the estimated impulse response of the transmission channelin a frequency domain as an initial coefficient, receiving a channelequalized signal fed back in data duration, calculating the errorbetween equalized signal and decision value of the same equalizedsignal, updating the coefficients by using the error between equalizedsignal and decision value of the same equalized signal continuously,compensating distortion of the received signal converted into thefrequency domain, and converting the compensated received signal backinto a time domain.
 24. The channel equalizer of claim 23, wherein thechannel estimator detects a training period, calculates across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,and outputs the calculated cross-correlation as the estimated impulseresponse of the transmission channel.
 25. The channel equalizer of claim23, wherein the channel estimator detects a training period, calculatesa cross-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,multiplies the cross-correlation and an inverse matrix of anauto-correlation matrix of the training signal, and outputting themultiplication result as the estimated impulse response of thetransmission channel.
 26. The channel equalizer of claim 23, wherein thechannel estimator detects a training period, calculates across-correlation between a training signal past through the channelduring the training period and a training signal preset by a receiver,multiplies the cross-correlation and an inverse matrix of anauto-correlation matrix of the training signal, and outputs an averageof the multiplication result and the estimated impulse response of aprevious frame of the transmission channel as the estimated impulseresponse of the transmission channel of the current frame.
 27. Thechannel equalizer of claim 23, wherein the channel distortioncompensator comprises: a first fast Fourier transform (FFT) unit forconverting the received signal from the time domain into the frequencydomain; a second FFT unit for converting the impulse response of thetransmission channel estimated by the channel estimator from the timedomain into the frequency domain; a ROM for in advance tabling andstoring frequency responses corresponding to an inverse channel of thetransmission channel of the frequency domain, and selectively outputtinga frequency response corresponding to an inverse channel of theestimated transmission channel, which was outputted from the second FFTunit; and a frequency domain equalizer for receiving the frequencyresponse of the inverse channel outputted from the ROM, setting thefrequency response as an initial coefficient, receiving a channelequalized signal fed back in data duration, calculating the errorbetween equalized signal and decision value of the same equalizedsignal, updating the coefficients by using the error between equalizedsignal and decision value of the same equalized signal continuously,compensating the distortion of the received signal converted into thefrequency domain, and converting the compensated received signal backinto the time domain.
 28. The channel equalizer of claim 27, wherein thefirst FFT unit overlaps a received data block whose length is M and aprevious data block, and converts the received data block into thefrequency domain.
 29. The channel equalizer of claim 27, wherein thesecond FFT unit augments zeros to the impulse response of the estimatedchannel, whose length is M, so that a size of the augmented impulseresponse matches a size N of an FFT block, and converts the augmentedimpulse response into the frequency domain.
 30. The channel equalizer ofclaim 27, wherein the frequency domain equalizer comprises: acoefficient bank for receiving the frequency response of the inversechannel outputted from the ROM, setting the frequency response as aninitial coefficient for frequency domain equalization, and storing andoutputting coefficients that are updated continuously in the dataduration; a first complex multiplier for multiplying the receivedfrequency domain signal outputted from the first FFT unit and acoefficient outputted from the coefficient bank and compensating thechannel distortion contained in the received frequency domain signal; anIFFT unit for converting the received frequency domain signal, which isoutputted from the first complex multiplier and from which distortionwas compensated, back into the time domain; a third FFT unit forreceiving as an error signal a difference between an output of the IFFTunit and the signal from which a noise was eliminated by the noisecanceller, and converting the error signal into the frequency domain; acomplex conjugate generator for outputting complex conjugate values ofthe received frequency domain signal outputted from the first FFT unit;a second complex multiplier for multiplying an output of the third FFTunit and an output of the complex conjugate generator; a multiplier formultiplying an output of the second complex multiplier and a step size(α); and an adder for adding an output of the multiplier and a previouscoefficient fed back from the coefficient bank thereby updating acoefficient, and outputting the updated coefficient to the coefficientbank.
 31. The channel equalizer of claim 30, wherein the frequencyresponse of the inverse channel is inputted to the coefficient bankwhenever the training signal is inputted.
 32. The channel equalizer ofclaim 30, wherein the frequency response of the inverse channel is earlyinputted only once to the coefficient bank, and then channel equalizeddata is fed back to update the coefficient.
 33. The channel equalizer ofclaim 30, wherein the IFFT unit extracts only rear M samples from Nsignals converted into the time domain and outputs the extracted rear Msamples to the noise canceller.
 34. The channel equalizer of claim 30,wherein the third FFT unit augments zeros to a front of the error signalwhose length is M so that a size of the augmented error signal matches asize N of an FFT block, and converts the augmented error signal into thefrequency domain.
 35. The channel equalizer of claim 30, furthercomprises: a noise canceller for predicting a noise from an output ofthe channel distortion compensator, the noise being amplified whenequalized, and eliminating the amplified noise contained in the timedomain signal outputted from the channel distortion compensator.
 36. Adigital television receiver comprising: a demodulator for digitalizing areceived signal and demodulating the digitalized signal into a base bandsignal; a channel estimator for estimating an impulse response of atransmission channel from an output signal of the demodulator; a channeldistortion compensator for converting the received base band signal andthe estimated impulse response of the transmission channel intofrequency domain signals, setting a reciprocal of the estimated impulseresponse of the transmission channel in a frequency domain as an initialcoefficient, receiving channel equalized data fed back in data duration,calculating the error between equalized signal and decision value of thesame equalized signal, updating the coefficients by using the errorbetween equalized signal and decision value of the same equalized signalcontinuously, compensating distortion of the received signal convertedinto the frequency domain, and converting the compensated receivedsignal back into a time domain; a noise canceller for predicting a noisefrom an output of the channel distortion compensator, the noise beingamplified during equalization, and eliminating the amplified noisecontained in the time domain signal outputted from the channeldistortion compensator; and an error compensator for compensating aphase and an error of data outputted from the noise canceller andoutputting the compensated data for channel decoding.
 37. The digitaltelevision receiver of claim 36, wherein the channel distortioncompensator comprises: a first fast Fourier transform (FFT) unit forconverting the received signal from the time domain into the frequencydomain; a second FFT unit for converting the impulse response of thetransmission channel estimated by the channel estimator from the timedomain into the frequency domain; a ROM for in advance tabling andstoring frequency responses corresponding to an inverse channel of thetransmission channel of the frequency domain, and selectively outputtinga frequency response corresponding to an inverse channel of theestimated transmission channel, which was outputted from the second FFTunit; and a frequency domain equalizer for receiving the frequencyresponse of the inverse channel outputted from the ROM, setting thefrequency response of the inverse channel as an initial coefficient,receiving a channel equalized data fed back in data duration,calculating the error between equalized signal and decision value of thesame equalized signal, updating the coefficients by using the errorbetween equalized signal and decision value of the same equalized signalcontinuously, compensating the distortion of the received signalconverted into the frequency domain, and converting the compensatedreceived signal back into the time domain.
 38. The digital televisionreceiver of claim 37, wherein the frequency domain equalizer comprises:a coefficient bank for receiving the frequency response of the inversechannel outputted from the ROM, setting the frequency response as aninitial coefficient for frequency domain equalization, and storing andoutputting coefficients that are updated continuously in the dataduration; a first complex multiplier for multiplying the receivedfrequency domain signal outputted from the first FFT unit and acoefficient outputted from the coefficient bank and compensating thechannel distortion contained in the received frequency domain signal; anIFFT unit for converting the received frequency domain signal, which isoutputted from the first complex multiplier and from which distortionwas compensated, back into the time domain; a third FFT unit forreceiving as an error signal a difference between an output of the IFFTunit and the signal from which a noise was eliminated by the noisecanceller, and converting the error signal into the frequency domain; acomplex conjugate generator for outputting complex conjugate values ofthe received frequency domain signal outputted from the first FFT unit;a second complex multiplier for multiplying an output of the third FFTunit and an output of the complex conjugate generator; a multiplier formultiplying an output of the second complex multiplier and a step size(α); and an adder for adding an output of the multiplier and a previouscoefficient fed back from the coefficient bank thereby updating acoefficient, and outputting the updated coefficient to the coefficientbank.
 39. The digital television receiver of claim 36, wherein the noisecanceller comprises: a noise predictor for extracting only colored noisefrom an output of the channel distortion compensator by using the outputof the channel distortion compensator and a decision value of a signalfrom which an amplified noise is eliminated and which is fed back, andpredicting the noise amplified during equalization; and a firstsubtracter for subtracting the noise predicted by the noise predictorfrom the output of the channel distortion compensator, thereby whiteningthe noise.
 40. The digital television receiver of claim 39, wherein thenoise canceller further comprises: a decision unit connected to anoutput terminal of the noise canceller, for outputting a decision valuenearest to a signal which is outputted from the noise canceller and fromwhich the amplified noise is eliminated; a multiplexer for feeding thetraining signal back to the noise predictor in a training period andfeeding back the decision value nearest to the signal from which thenoise was eliminated to the noise predictor in a data duration; and asecond subtracter for outputting as an error signal a difference betweena signal outputted through the multiplexer and an output signal of thechannel distortion compensator to the third FFT unit of the channeldistortion compensator.